Method and device for continuously monitoring an optical transmission path

ABSTRACT

The main steps of the invention comprise feeding a sample signal (S OTDR ) into the transmission line or path ( 2 ) using a multiplexer ( 8 ) during a current signal transmission and measuring the intensity of said sample signal (S OTDR ), which is also coupled out via the multiplexer ( 80  and is reflected partially at faulty or error points. Part of the signal to be transmitted (S in ) can also be deviated by means of a coupling device ( 4 ) and the intensity of this proportion of the signal (S up ) can be compared with a threshold value. By evaluating the intensities that are measured, it is possible to detect and localize defects in the transmission path ( 2 ) or judge the quality of the transmission path ( 2  without interrupting the transmission.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 ofGerman Patent Application No. 19949401, filed Oct. 13, 1999, and is anational stage filing, under 35 U.S.C. § 371 of PCT ApplicationPCT/DE00/03618, filed Oct. 13, 2000.

FIELD OF THE INVENTION

The invention is directed to a method and device for continuousmonitoring an optical transmission path or line.

BACKGROUND OF THE INVENTION

Data traffic on optical transmission lines is constantly increasingowing to the increase in the channel-bit width in time multiplex and thenumber of channels in wavelength multiplex. Because of the increasedload on transmission lines, errors are being increasingly noticed in thetransmission line and lead to transmission errors. A demand thereforeexists for effective methods and devices to monitor optical transmissionlines.

At the present time, when there are problems on a transmission line, thetransmission lines are generally briefly interrupted in order to be ableto connect corresponding measurement devices by which a transmissionsignal can be coupled into the transmission line in order to test thetransmission line. A fraction of the transmitted signal can also becoupled out of line in order to test the transmission line. Continuousor at least uninterrupted coupling is not possible here.

A monitoring system already exists, available from the CIENA Corporationunder the name WaveWatcher®, which permits continuous monitoring of anoptical network. However, this is a relatively costly system with anetwork element manager, an optical service channel and an elementmanagement system embedded in an optical network. The system monitors,measures and stores the status and operation method of each module inthe system. A standard data communications network is used, which issupported by the optical service channel, in order to process managementinformation about the system. The data communications network permitsremote access and remote control of network management elements. Thedrawback of this system is that it is relatively costly.

A system for monitoring optical networks is also available from LucentTechnologies, which is used to supervise the configuration, performance,reliability and other parameters of an optical network at the networkmanagement level. This system determines the components that areinvolved when an error occurs in the network. The operator can thendecide what should be done with these components, and he can inform theuser of these circuits. This system is also relatively expensive.

As a result, the underlying objective of the present invention is toprovide continuous monitoring of an optical transmission line or anoptical network by the simplest possible means.

SUMMARY OF THE INVENTION

In one aspect of the invention, a method for monitoring of an opticaltransmission line is provided in which a test signal is coupled into themonitored transmission line without interruption of the monitored line.The test signal reflected by the transmission line is coupled outwithout interruption of the monitored line, the frequency of the testsignal is chosen to be outside of the transmission band of thetransmitted signal, and an assessment of the quality of the transmissionline, or an error in the transmission line, is made by measurement ofthe reflected test signal. Since no interruption of the transmittedsignal is required for coupling in or coupling out of the test signal, acontinuous and uninterrupted monitoring of the optical transmission linecan be easily be carried out with the invention.

One embodiment of the method according to the invention is that the testsignal is an OTDR signal. OTDR (optical time domain reflectometer) is asystem used to determine the quality of transmission lines or errors ina transmission line, in which the signals reflected by the transmissionline are measured. In the event of an error, location resolution can beachieved by means of reflected signals, which means assessmentsconcerning the location and type of error can be made via thepropagation time of the test signal from the coupling to the reflectionsite and back out of the coupling and via the intensity of the reflectedsignal.

The aforementioned objective is further realized by a method formonitoring of optical transmission lines, in which a fraction of atransmitted or incoming signal (S_(IN)) is coupled out withoutinterruption of the monitored line, and that an assessment of thequality of the transmission line, or an error in the transmission line,is made by measurement of the coupled out signal (S_(SUP)). In thismethod; continuous and uninterrupted monitoring of the opticaltransmission line is possible, in which the transmitted signal itself,that is, a fraction of it, is directly used to measure the quality ofthe transmission line.

It is advantageous if the method just mentioned is applied incombination with the first-named method because assessments can then bemade about the quality of the transmission line or errors in thetransmission line, such as an increasing drop in transmission power or athreatening failure of the transmission line, from both measuredquantities, namely, the reflected signal of the test signal and thecoupled out fraction of the transmitted signal. Measurement of thetransmitted signal gives a direct indication of the interaction betweenthe transmission line and the transmitted signal, whereas evaluation ofthe test signal permits location resolution; that is, localization ofthe error, in the event of an error.

A further embodiment of the method according to the invention is thatthe transmission signal (S_(SUP)), i.e., the coupled out fraction of thetransmitted signal (S_(IN)), is filtered before measurement: so thatonly signals lying in the transmission band of the transmitted signal(S_(IN)) are passed through. Undesired signal fractions and, in theevent of a test signal, their inclusion in the measurement of thetransmitted signal, are therefore avoided so that exact measurement ofthe interaction of the transmission line with the transmitted signal canoccur. In addition, the saturation or destruction/damage to the receiverby the OTDR signal is avoided, which is known to have a high outputpower.

Another embodiment of the method according to the invention is that themeasurement result is processed according to a logic, and that an alarmis issued when an error is found. Processing of the measurement resultvaluable to the user occurs because it enable the user can reactaccordingly.

Another embodiment of the method according to the invention ischaracterized by the fact that the intensity of the coupled out signallies at 1-10% of the intensity of the transmitted signal. The totalpower of the transmitted signal that is further transmitted duringexecution of the method according to the invention is thereforeadvantageously only slightly reduced.

Finally, another embodiment of the method according to the invention isthat the test signal is coupled into the transmission line of theincoming and/or outgoing transmitted signal. Monitoring of thetransmission line can therefore be carried out both opposite thedirection of the transmitted signal and in the direction of thetransmitted signal. When both monitoring capabilities are exploited, acost savings is produced from the fact that the modules for execution ofthe method according to the invention need only be inserted after everyother transmission line unit is supervised.

To realize the aforementioned objective, a device for the monitoring ofoptical transmission lines according to the invention is characterizedby a multiplexer, via which a test signal (S_(OTDR)), whose frequencylies outside of the transmission band of the transmitted signal, iscoupled into the transmission line without interruption of the monitoredline, or a reflected signal is coupled out from the line, and by acoupler, with which the reflected signal is coupled out from thetransmission line without interruption of the monitored line, in which,an assessment is made concerning the quality of the transmission line oran error in the transmission line by use of the measurement of thereflected test signal. For implementation of a module for execution ofthe method according to the invention, only a multiplexer and a couplerare therefore required, and the transmission line can be continuouslymonitored without interruption.

An additional embodiment of the device according to the invention ischaracterized by the fact that an OTD reflectometer is connecteddownline of the coupler. With this arrangement, a conclusion can be madefrom the reflected test signal concerning the location of an error sitein the transmission line.

Another device to realize the aforementioned objective is characterizedby the use of a coupling device through which a fraction of atransmitted or incoming signal (S_(IN)) is coupled out as monitoringsignal (S_(SUP)), a measurement device to measure the coupled out signal(S_(SUP)), and a device through which an assessment is made concerningthe quality of the transmission line or an error in the transmissionline based on the measured signal. Here, only one coupling device andone measurement device are advantageously required in order to performthe measurements that are necessary for continuous and uninterruptedmonitoring of the transmission line.

A major advantage of the device according to the invention lies in theoptimal combination of the components WIC (wavelength independentcoupler) and WDM (wavelength division multiplexer) in one module thatsupplies the required function. The module need only be inserted in anexisting transmission system.

An advantageous embodiment of the device is that the multiplexer is aWDM. In such a multiplexer, it is advantageous that signals of anyfrequencies, that even signals with frequencies outside of thetransmission band of the transmitted signal, can be coupled into thetransmission line.

An advantageous embodiment of the device according to the invention isthat the measurement device is a photodetector that measures the powerof the coupled out transmitted signal.

An advantageous embodiment of the device according to the invention isthat the device for evaluation of the measured, coupled out transmittedsignal is a logic circuit with an alarm function.

Another advantageous embodiment of the device according to the inventionis that the coupling device is a so-called WIC device, so that themodule can be used over a wide frequency range of transmitted signals.

Another advantageous embodiment of the device according to the inventionis that an optical bandpass filter is provided between the couplingdevice and the measurement device in order to filter out signalfractions that are not supposed to be measured in the measurementdevice. Especially, when a test signal is used in addition to thetransmitted signal, the test signal must be filtered out in order topermit precise measurement of the transmitted signal. Saturation ordestruction or damage to the receiver by the OTDR signal is alsoavoided, which is known to have high output power.

Another advantageous embodiment of the device according to the inventionis that two wavelength multiplexers are provided. One wavelengthmultiplexer couples the test signal for the incoming part of thetransmission line, and the other wavelength multiplexer couples the testsignal (S_(OTDR,out)) for the outgoing part of the transmission line.Both parts of the transmission line can thus be advantageously monitoredby one module.

Another advantageous embodiment of the device according to the inventionis that the multiplexer is arranged in the transmission direction infront of the coupler, in which filtering of the OTDR signal can bedispensed with. Finally, an additional advantageous embodiment of thedevice according to the invention is that the devices that form a modulefor monitoring optical transmission lines are arranged on a printedcircuit or on a plug-in unit. In this manner, the module can beintegrated in existing hardware without additional means.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention are now described with reference to theaccompanying drawings. In the drawings:

FIG. 1 illustrates the layout of a module for supervision of opticaltransmission lines when the incoming branch of a transmission line is tobe supervised;

FIG. 2 illustrates the layout of a module for supervision of opticaltransmission lines when the outgoing branch of a transmission line is tobe supervised;

FIG. 3 illustrates the layout of a module, as in FIG. 1, with theaddition of a photodetector and a logic circuit to generate an alarmsignal;

FIG. 4 illustrates the layout of a module, as in FIG. 1, with theaddition of a photodetector and a logic circuit to generate an alarmsignal with an additional optical filter;

FIG. 5 illustrates the layout of a module, when the OTDR signal iscoupled into both the incoming and outgoing transmission lines;

FIG. 6 illustrates the layout of a module for supervision of opticaltransmission lines as an alternative to FIG. 1; and

FIG. 7 illustrates a plug-in unit with a module according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a module for monitoring of optical transmission lines,especially a transmission line 2 for an incoming signal S_(IN). Theincoming signal S_(IN) is fed to the gate T1 (the input signal arm) of awavelength-independent coupler 4 (WIC=wavelength-independent coupler),which couples out a fraction of the incoming signal S_(IN) at gate T2(the tap arm), while the remaining main part of the incoming signal SINis coupled out at gate T3 (the signal out arm). The power fraction ofthe incoming signal SN coupled out at gate T2 lies in the range from 1to 10% of the signal S_(IN) coupled in at gate T1. The main fraction ofthe incoming signal S_(IN) is released via a transmission line 6 to afirst wavelength multiplexer 8 (WDM=wavelength division multiplexer).The multiplexer 8 receives the signal at gate T4 (the common arm). AnOTDR signal S_(OTDR) is coupled in at gate T5 (the test signal arm) ofthe wavelength division multiplexer 8 into the signal transmission lineor coupled out from it. At a gate T6 (the communication signal out arm),the output signal S_(out) from the module is coupled out and releasedvia a transmission line 10. The signal S_(OTDR) is evaluated in an OTDrefractometer 12. The wavelength of signal S_(OTDR) lies outside thetransmission band of the transmitted signal. If the signal istransmitted in the third optical window (at about 1550 nm), the signalS_(OTDR) can lie at 1310 nm or above 1600 nm (typically at 1625 nm). Ifit is transmitted in the second optical window, the signal S_(OTDR) liesin the wavelength range above 1550 nm or at 1625 nm.

FIG. 2 shows the essential layout of a module for monitoring of theoutgoing branch of the transmission line. The same reference numbers areused in FIG. 2 for the same parts as shown in FIG. 1. By reversing thewavelength multiplexer 8, a situation is achieved in which a signalS_(OTDR) coupled in at gate T5 has a travel direction so that theoutgoing branch of the transmission line or the transmission line 10 issupervised by signal S_(OTDR). The behavior of the transmission line 10with reference to the transmitted signal S_(out) is then monitored inthe next monitoring module in the transmission line, where a WIC is alsoprovided, which then couples out part of the transmitted signal,permitting conclusions concerning the transmission characteristics oftransmission line 10.

FIG. 3 shows the module of FIG. 1, in which a photodetector and a logiccircuit 16 are added. The photodetector 14 receives the coupled outfraction S_(SUP) of the transmitted signal S_(IN) and sends acorresponding measured value to the logic circuit 16, which, forexample, via a threshold value determination, indicates the quality ofthe transmission line or establishes an error in the transmission lineand issues an electrical signal or alarm signal.

FIG. 4 is another advantageous embodiment of the module according to theinvention, in which a bandpass filter 18 is provided in front of thephotodetector 14, so that the back-scattered signal S_(OTDR) does notfall on the photodetector 14 and distort the measurement there or oversaturate or damage the detector.

The variants according to FIGS. 3 and 4 equally apply to the practicalexamples of FIGS. 1 and 2.

FIG. 5 shows the essential layout of a module when the OTDR signal iscoupled into the incoming and outgoing transmission lines. For thispurpose, a wavelength-dependent coupler 30, a first wavelengthmultiplexer 32 and a second wavelength multiplexer 34 are provided inthe transmission line 2. As in the preceding practical examples, part ofthe signal S_(IN) that arrives at gate T1 of coupler 30 is coupled outat gate 12 of the coupler 30 as supervision signal S_(SUP).

The transmission signal is released from gate T3 of coupler 30 to gateT4 of the first wavelength multiplexer 32. At gate T5 of the wavelengthmultiplexer 32, the test or OTDR signal S_(OTDR,in) is coupled in, whichis supposed to supervise the incoming part of the supervision line. Thetransmitted signal is released at gate T6 of the wavelength multiplexer32 and introduced at gate T7 to the second wavelength multiplexer 34.The transmitted signal is released at gate T8 of the wavelengthmultiplexer 34 as signal S_(out). A test signal S_(OTDR,out) is coupledin at gate T9, which is supposed to monitor the outgoing part of thetransmission line. The coupler 30 and the wavelength multiplexer 32, 34can again be assembled in one module.

FIG. 6 shows the essential layout of a module for the monitoring of anoptical transmission line, in which the same components are used as inthe practical example of FIG. 1. Departing from FIG. 1, however, thewavelength multiplexer 8 here is arranged in the transmission directionin front of coupler 4, which is shown by signals S_(IN) and S_(OS)[sic].The test signal S_(OTDR) coupled into the wavelength multiplexer 8serves to monitor the incoming part of the supervision line. Thispractical example of the invention has the additional advantage thatfiltering of the OTDR signal can be saved because this signal is coupledout before coupler 4 and therefore cannot reach the detector.

The entire module can be constructed from discrete individual componentsor be an optically-integrated component based on SiO₂, Si or InPtechnology. Overall damping of the signal between gate T1 and gate T6 isless than 3 dB.

FIG. 7 shows a plug-in unit with a circuit board 20, on which themonitor module 24, a panel with OWG couplings 22, especially couplingsfor channels 1 to 4, an output 26 for an electrical alarm or a powersupply and a network interface 28 are included. The network interface isadvantageous in that the computer capacity of the computer can beutilized via this plug-in unit, in which the plug-in unit is installedin order to perform evaluations of signals measured in the monitoringmodule (OWG=optical wave guide).

Although in the preceding description the monitor module is described sothat it has a wavelength-independent coupler 4 or 30 in combination withone or more wavelength multiplexers 8 or 32, 34, it should be noted thatthe monitoring functions of these two components could be usedseparately.

1. A method for continuously monitoring an optical transmission line,said method comprising the steps of: a extracting a faction of atransmitted or received signal S_(IN) independently of wavelengthwithout interrupting the line being monitored, introducing a test signalinto the line being monitored without interrupting the line beingmonitored, and extracting the test signal reflected by the transmissionline without interrupting the line being monitored; wherein thefrequency of the test signal is chosen to be outside the transmissionband of the transmitted signal, and wherein information on the qualityof the transmission line, or an error or defect in the transmissionline, is obtained based on a measurement of the extracted signalfraction (S_(SUP)) as well as the reflected test signal.
 2. The methodaccording to claim 1, wherein the test signal S_(OTDR) is an opticaltime domain reflectometer signal.
 3. The method according to claim 2,wherein S_(SUP), being an extracted fraction of the transmitted signalS_(IN), is flittered before the measurement in such a way that onlysignals that lie in the transmission band of the transmitted signalS_(IN) are allowed to pass.
 4. The method according to claim 3, whereinthe measuring result is logically processed and an alarm is triggered ifan error or defect is detected.
 5. The method according to claim 3,wherein the power of the extracted signal fraction S_(SUP) is in therange of 1-10% of the power of the transmitted signal S_(IN).
 6. Themethod according to claim 5, wherein the test signal is introduced intothe transmission line of the signal being received or of the signalbeing transmitted.
 7. The method according to claim 1, wherein theS_(UP), being an extracted fraction of the transmitted signal S_(IN), isfiltered before the measurement in such a way that only signals that liein the transmission band of the transmitted signal S_(IN) are allowed topass.
 8. The method according to claim 1, wherein the power of theextracted signal fraction (S_(SUP)) is in the range of 1-10% of thepower of the transmitted signal S_(IN).
 9. The method according to claim1, wherein the test signal is introduced in to the transmission line ofthe signal being received of the signal being transmitted.
 10. A devicefor continuously monitoring an optical transmission line said devicecomprising: a coupler having an arm for receiving an input signal froman optical transmission line, a tap arm for separating a faction of theinput signal, and a signal output arm, and a first multiplexer having acommon arm, a test signal arm for receiving a test signal from a testsignal source, and a communications signal output arm, and saidmultiplexer being further capable of extracting a reflected signal fromsaid transmission line; wherein said coupler separated fraction of saidinput signal and said multiplexer extracted signal are sent to separatemeasuring and/or evaluating devices to obtain information regarding thequality of the transmission line and/or defects in the transmissionline, and said coupler signal output arm and multiplexer common arm areconnected to enable the passage of an optical signal between the two,and the test signal frequency is one which lies outside the frequenciesbeing transmitted in said optical transmission line, and said allsignals are coupled into or out of said mission without interrupting thesignal passing through said transmission line.
 11. The device accordingto according to claim 10, wherein the evaluating device for the signalextricated from the multiplexer is an optical time domain reflectometer.12. The device according to claim 10, wherein the measuring device forthe signal from the coupler is a photodetector.
 13. The device accordingto claim 12, wherein a logic circuit with an alarm is connected to saidphotodetector to evaluate the measured signal transmitted therefrom. 14.The device according to claim 13, wherein a bandpass filter is arrangedbetween the coupling device and the measuring device.
 15. The deviceaccording to claim 12, wherein a bandpass filter is arranged between thecoupler and the photodetector.
 16. The device according to claim 10,wherein a bandpass filter is arranged between the coupler and aphotodetector.
 17. The device according to claim 10, wherein said devicecontain a second multiplexer in addition to said first multiplexer andsaid coupler, wherein one of said multiplexer couples a test signal intothe incoming section of the optical transmission line, and the otherwavelength multiplexer couples a test signal into the outgoing sectionof the optical transmission line.
 18. The device according to claim 10,wherein, in the direction of optical transmission, the first multiplexeris arranged before the coupler.
 19. The device according to claim 10,wherein the device is a module which can be mounted on a printed-circuitboard or on a rack.